Rigorous biochemical analyses of pulmonary surface active material show that it contains several lipids, proteins and carbohydrate. Despite this complex composition, this material can be isolated from several species with a relatively fixed stoichiometry among its constituents. Arguments based on physico-chemical and physiological studies indicate that surfactant must contain these several lipid constituents to fulfill its physiological functions. We will investigate one means by which the stoichiometry of the lipid constituents may be regulated. Since we have previously found that the apoprotein in surfactant interacts strongly with certain lipids, we will carry out binding studies between the apoprotein and certain lipids to see if the apoprotein has particularly strong affinities for particular lipids, and if the binding properties might control the composition of lipoproteins formed by recombination. The results may provide information on how lipids necessary for physiological function interact physico-chemically. They may also direct attention to certain components which comprise a relatively small proportion of the total composition of surfactant, but which may have a physiological role disproportionate to their absolute content.